1. Field of the Invention
This invention relates generally to circuits for providing a high frequency energizing signal to electrical energy utilization devices.
2. Description of the Prior Art
The traditional measure of the efficiency of energy utilization in luminescent sources is a parameter called "efficacy", which is the ratio of luminous flux output (lumens) to total power input (watts). For example, the efficacy of present day fluorescent tubes is about 55 to 65 lumens per watt as compared to a figure of about 40 lumens per watt for typical incandescent lamps. Solely from the standpoint of energy utilization efficiency, therefore, it would seen desirable to employ fluorescent lamps for most lighting needs.
However, as relatively efficient as they are when compared with some light sources, present day fluorescent lamps fall far short of the efficiencies theoretically possible. Fluorescent lamps, just as do all gaseous-discharge tubes, require a high voltage to initiate current flow across the lamp terminals. This is due to the fact that there is an infinitely high impedance existing in the tube prior to ignition. Ignition occurs when the gases inside the tube are ionized, thus permitting current to flow between the electrodes at each end of the tube. Once a gaseous-discharge tube has ignited, it exhibits a negative resistance characteristic, and, some sort of current control device is typically utilized to limit the current to the tube. These current limiting devices are referred to as "ballasts."
Typically, a fluorescent lamp ballast includes circuitry adapted to direct a high voltage (which may be as high as 1000 volts) to the gas tube electrodes. This high voltage is necessary in order to force electron emission from those electrodes and thereby initiate ionization of the gases in the tubes. One or both of the electrodes generally comprises a filament which has the capacity of more readily emitting electrons when heated and subjected to such high voltage.
One disadvantage with a present day mercury vapor fluorescent lamp circuit involves the loss of energy in the operation of the ballast and in the heating of the filament electrodes. Another disadvantage is that the lifetime of the lamps is controlled principally by the mechanical integrity of the filaments. Once the filaments break and cease to emit electrons the lamps no longer function, even though the light producing components of the lamp (the gases in the tube and the phosphorus on the tube walls) remain functional.
It is generally acknowledged that the energization of fluorescent tubes with high frequency signals is more efficient than standard ballast circuits. For one reason or another, however, these systems have not been commercially feasible. Apparently, in prior art circuits too much energy is lost in the switching and amplification of transistors and in the operation of the power transformer. Another factor which may have been encountered is the fact that bulb life is greatly reduced and the ends of the tube tend to become blackened due to current distortions in the tube caused, at times, by the introduction into the tube of signals carrying too many harmonics.